Beat the Book
Automotive electrical diagnosis.
Better diagnosis, faster repair.
Application Note
Work safely
This application note is intended
as a guide for the professional
mechanic. It describes some of
the test procedures commonly
used by experienced technicians.
However, some of the procedures
require you to take certain pre-
cautions to avoid personal injury
and/or damage to equipment or
vehicles. Fluke cannot anticipate
all possible precautions that you
must take on all the different
vehicles for which this guide is
applicable. For this reason, you
should thoroughly familiarize
yourself with proper safety
habits and take all precautions
described in the vehicle’s
repair manuals to avoid injury
or damage.
Testing automotive
systems with an
analog/digital multimeter
Perhaps the most important tool
you’ll use in troubleshooting auto
electrical systems is the multime-
ter. The basic multimeter
measures voltage, current and
resistance, while more elaborate
multimeters, such as the new
Fluke 88-V, have features that
can check things such as
frequency, duty cycle, pulse
width, make diode tests, and
even measure temperature.
In this application note you
will find troubleshooting tech-
niques using the high quality,
affordable and popular line of
Fluke analog/digital multimeters
and automotive troubleshooting
accessories. Fluke’s analog/
digital combination has distinct
advantages over either digital
or analog meters alone, and
the accessories are designed to
make your job easier and more
profitable.
Although automotive multime-
ters with various capabilities
have been around for years,
many of them didn’t have the
scales and functions required for
todays’ automotive applications.
Hybrid, electric and fuel cell
powered vehicles have special
requirements, ones met with the
new 88-V. Now that computers
and their sensors are a part of
everyday automobile trou-
bleshooting, you need a true
multimeter to do the job.
The common analog type mul-
timeter is not only inadequate, it
can damage delicate computer
circuitry. Both analog meters and
test lights, due to their low inter-
nal resistance (input impedance),
draw too much power from the
device they’re testing to be used
on computers. What’s more,
many analog meters use 9V to
power the resistance test, which
is enough to destroy sensitive
digital components.
From the Fluke Digital Library @ www.fluke.com/library
Over the last couple decades, a
different type of multimeter has
evolved to solve problems not
addressed by analog multimeters.
Digital multimeters (DMMs) have
much higher input impedance
than analog meters, generally
1 Meg ohms (million ohms) to
10 Meg ohms. The high imped-
ance means that the meter will
draw very little power from the
component under test. Besides
providing more accurate meas-
urements, this type of meter will
not damage sensitive computers.
The one problem with digital
readouts has been that the num-
bers displayed didn’t give much
information about whether the
reading was increasing or
decreasing. You may have experi-
enced the frustration of trying to
read the constantly changing
numbers on digital exhaust gas
analyzers or scan tools.
Fluke has overcome the prob-
lems associated with traditional
analog and digital meters by
providing a combination display
that gives you the accuracy of a
digital readout with the dynamic
measurement capabilities of an
analog meter. Rather than adding
new meters to test every gizmo
that comes along, one good
multimeter will suffice.
Thinking about
troubleshooting
When troubleshooting electrical
systems, it’s important to use a
logical process of deductive
reasoning to solve the problem.
This process is most important
since you can’t see inside or dis-
mantle the majority of electrical
components to tell whether
they’re functioning, as you can
with mechanical devices.
Jumping to conclusions can be
expensive and time consuming.
With well thought-out and
organized steps, you can usually
determine the source of the prob-
lem the first time.
The key tool in this process is
the DMM.
Types of measurements
When troubleshooting auto-elec-
trical systems, you measure
voltage, current and resistance.
Probably the easiest and most
useful measurement is voltage. It
can answer these questions:
•
Is voltage present?
•
What is the voltage reading?
•
What is the available voltage?
•
What is the voltage drop
across a component or
connector?
The presence of voltage tells you
that the circuit is delivering elec-
tricity to the component you’re
testing.
The voltage reading tells you
whether the proper voltage is
arriving at the component.
By measuring available voltage
at a component, you can deter-
mine whether the voltage
reaching the device is adequate.
The voltage drop across a
component tells you how much of
the voltage is being consumed by
that component.
For example, if a relay has
12.8 volts on the input side and
only 9.2 volts on the output side,
the voltage drop is said to be 3.6
volts. Remember that wire and
connections can also be consid-
ered components and may cause
voltage drops if faulty.
The Fluke 88-V Analog/Digital
Multimeter has several features
that make it well suited for
automotive troubleshooters:
Analog/Digital Display —
Combined 33-segment analog
bar graph and 6000-count digital
display.
Peak Min/Max —
The Peak
Min/Max function stores the low-
est and highest measurements
made by the meter as fast as
250 µS.
Auto Hold —
The meter display
“freezes” a reading until you’re
ready to look at it.
Automatic Polarity and Range
Selection —
The meter automati-
cally selects the range and
polarity for the best possible
reading.
Continuity Beeper —
Audible
“beep” tone provides easy testing
for continuity, shorts and diode
tests.
Rugged Construction —
Tough
plastic case survives the
everyday hazards of working
in a shop. Meter is backed by a
limited lifetime warranty.
2 Fluke Corporation
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mS Pulse Width
% Duty Cycle
Temperature
Pressure***
Diode Test
Continuity
Vacuum***
Amps DC*
Millivolts
Automotive electrical problems
can be divided into several cate-
gories, according to the system
causing the trouble. Remember
that the real problem may exist in
one system, while the symptoms
you’re testing appear in another.
The systems covered in this bul-
letin are:
•
Charging System
•
Starting System
•
Fuel/Air System
•
Ignition System
•
Body Electric/Engine
Management/Cooling Systems
Many people won’t bring a car
into the shop until it won’t start,
so for a great many failures that’s
the first symptom you’ll see. The
first challenge is to determine
which system is the cause of the
“no-start.”
Other failures result in chronic
problems of some sort, like a
repeatedly discharged battery or
hard starting when hot. Once
you’ve decided on the system
most likely to be the cause, you
can proceed with your tests,
using Fluke analog/digital multi-
meters.
Volts AC
Volts DC
System/Component
Charging System
Alternators
Computerized regulators
Connectors
Diodes (ac ripple)
Diode rectifier
Regulators
Starting System
Battery
Connectors
Interlocks (neutral safety switch)
Solenoid
Starters
Fuel/Air System
Engine speed
Engine vacuum
Feedback carburetor
Fuel injectors (electronic)
Idle air motors
Fuel system pressure
MAF sensor
MAP & BP sensors
Throttle position sensors
Igniton System
Coils
Condensors (capacitors)
Contact sets
Distributor cap
Ignition modules
Body Electric
Compressor clutch
Lighting circuits
Relay & motor diodes
Transmissions
Engine Management
Computer temperature sensors
Connectors
Hall-type sensor
O
2
sensors
Oil pressure
Cooling Systems
Connectors
Fan motor
Radiator
Relays
Temperature switches
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RPM**
Ohms
Hz
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*Used with Fluke i410 or i1010 current clamp
**Used with the RPM80 Inductive Pickup accessory (included)
***Used with optional PV350 Pressure/Vacuum Module
3 Fluke Corporation
Beat the book
Bar Graph
Troubleshooting with
a Fluke analog/digital
multimeter
Measurement Type
Min/Max
Charging System
Batteries
Charging system problems often
come to you as a “no-start”
complaint. The battery will have
discharged and the starter won’t
crank the engine. The first step is
to test the battery and charge it if
necessary (Figure 1).
No-load test
Voltage
12.60 V to 12.72 V
12.45 V
12.30 V
12.15 V
Percent charge
100 %
75 %
50 %
25 %
Load Test @ 1/2 CCA Rating
8.5 V @ 0 °F (-18 °C)
8.8 V @ 30 °F (-1 °C)
9.4 V @ 50 °F (10 °C)
9.6 V @ 70 °F (21 °C)
Figure 1. Measuring system voltage
Bleed the surface charge from the battery by turning on the head-
lights for a minute. Measure the voltage across the battery terminals
with the lights off (see chart). When possible, individual cell specific
gravity should be checked with a hydrometer. A load test should be
done to indicate battery performance under load. Voltage tests only
tell the state of charge, not the battery condition.
Readings obtained at 80 °F (27 °C)
Alternators
A DMM’s accuracy and digital
display make regulator/alternator
diagnosis and adjustment easy.
Regulators
First determine if the system has
an integral (internal) regulator,
then whether it’s type A or B.
Type-A has one brush connected
to battery + and the other brush
grounded through the regulator.
Type-B has one brush directly
grounded and the other con-
nected to the regulator.
Next, isolate the problem to
alternator or regulator by bypass-
ing the regulator (full-fielding).
Ground Type-A field terminal.
Connect Type-B field terminal to
Battery +. If the system now
charges, the regulator is faulty.
Use a rheostat if possible.
Otherwise, just idle the engine
(lights on) so the voltage doesn’t
exceed 15 V.
Figure 2. Verifying a good alternator
The battery must be fully charged (see Figure 1). Run the engine and
verify that no-load voltage is 13.8-15.3 V (check as in Figure 1). Next,
load the alternator to rated output current with a carbon pile across
the battery. Run the engine @ 2000 RPM. Check the current with an
i410 or i1010 current clamp.
4 Fluke Corporation
Beat the book
AC
/D
C
60
T: 1mV
LAMP
0V
/A
OU
EN
i4
TP
T
10
C
U
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RR
CA
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A
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60 00V
0A
Alternator ac leakage
AC
/D
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RE
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OU
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1
TP
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Figure 3. Checking field current
Worn brushes limit field current, causing low alternator output. To test:
load unit as in Figure 2 and measure field current with current clamp or
use 10 A jack on DMM. Readings range from 3 to 7 amps. On integral
GM units: with alternator not turning, jump terminals together and con-
nect both to Batt + with DMM in series set to measure 10 amps. Field
current should be between 2 and 5 amps, higher current with lower
battery voltage. Control battery voltage by loading it with a carbon pile.
Figure 4. Checking ripple voltage
Ripple voltage or (ac voltage) can be measured by switching your DMM
to ac and connecting the black lead to a good ground and the red lead
to the “BAT” terminal on the back of the alternator, (not at the battery).
A good alternator should measure less than .5 V ac with the engine
running. A higher reading indicates damaged alternator diodes.
Figure 5. Alternator leakage current
To check alternator diode leakage, connect the multimeter in series
with the alternator output terminal when the car is not running.
Leakage current should be a couple of milliamps at most; more often,
it will be on the order of 0.5 milliamps. Use care when disconnecting
the alternator output wire; make sure the battery is disconnected first.
Connect the meter, then reconnect battery.
5 Fluke Corporation
Beat the book
A
ZE
R
O
CA
T
6
60 00V
0A
An alternator generates current
and voltage by the principles of
electromagnetic induction.
Accessories connected to the
vehicles charging system require
a steady supply of direct current
at a relatively steady voltage
level. You can’t charge a battery
with alternating current, so it
must be rectified to direct current.
60
T: 1mV
LAMP
0V
/A
CA
T
III
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